This invention relates to chelating resins which can be used to selectively remove heavy metal ions from aqueous solutions and to a process for preparing such resins.
One major problem in the chemical industry is the removal of an increasingly large number of pollutants from waste water and other process streams. Following the lead of the Federal Water Pollution Control Act of 1972 (commonly referred to as the "Clean Water Act"), many states, counties, and even cities are posting ever more stringent regulations concerning the discharge of a wide variety of polluting materials, both organic and inorganic. One group of materials causing particular concern is the "priority" pollutants as established under Section 307 of the Clean Water Act. These have been determined to present unusual hazards in terms of toxicity, carcinogenicity and/or mutagenicity. Presently included in this list are ions and salts of some 13 "heavy" metals which, along with the current federal drinking water and maximum allowable river discharge limits, are given in Table I.
TABLE I ______________________________________ Maximum Allowable Concentrations Pollutant Heavy Metal (ppm) ______________________________________ Antimony 0.15 Arsenic 0.05 Beryllium 0.000037 Cadmium 0.01 Chromium 0.05 Copper 1.0 Lead 0.05 Mercury 0.002 Nickel 0.013 Selenium 0.01 Silver 0.05 Thallium 0.013 Zinc 5.0 ______________________________________
In treating aqueous streams to remove these elements, it is found that these elements are not always found in cationic form. Rather, they may be incorporated into various anions, non-ionic complexes or bound in suspended particulates. Consequently, a plurality of treatments may be necessary to more or less completely remove them. This imposes a significant capital and operating cost penalty on any facility which is required to handle complex process chemicals in waste water streams, especially those having several of these pollutants present at the same time.
One approach to pollution abatement which appears to offer unusual cost effectiveness is the use of one or more chelating agents as absorption compounds. Capable of operating in a wide variety of chemical environments, such materials are finding wide use in treating process and waste water streams to selectively remove a considerable number of heavy metal compounds.
For this purpose, several different types of agents can be used, both inorganic and organic with several different treatment modes, including batch, fixed bed and fluidized bed processes being available. While some of these agents are quite specific in their selectivity, most are fairly general in this regard and are thus able to sequester and bind compounds of a large number of elements, often both in ionic and non-ionic form. This ability, while generally advantageous, can create a problem when solutions containing mixtures of "pollutants" and "non-pollutants" must be treated. To the extent that the non-pollutant is sequestered, the capacity of the agent for the pollutant or pollutants of interest is diminished. Since many of these agents can only be regenerated with great difficulty, if at all, such a problem can significantly shorten service life.
This situation arises, for example, in the chlor-alkali industry where the anolyte brines from mercury cells, after dechlorination and resaturation, nominally contain between about 1 and about 20 parts per million (ppm) of mercury and small amounts of other heavy metal contaminants such as iron, copper, lead and nickel. They also contain substantial amounts of non-pollutant sulfate, chlorate and alkaline earth metal ions such as calcium and magnesium, all in a concentrated brine of the alkali metal being electrolyzed. Where part of the brine is periodically purged, as for chlorate and sulfate control, the loss of the mercury dissolved therein frequently results in both a surprisingly large monetary loss as well as the creation of a significant environmental hazard.
One group of chelating agents which have showed unusual promise for selectively removing mercury and other heavy metals are the poly(dithiocarbamate) resins as reported by Hackett and Siggia of "Selective Concentration and Determination of Trace Metals Using Poly(dithiocarbamate) Chelating Ion-Exchange Resins" in Environmental Analysis, edited by G. W. Ewing, Academic Press, Inc., New York, New York, 1977, pages 253-265. These resins were found to sequester a large number of heavy metals while being essentially insensitive to the presence of alkali and alkaline earth metal ions in solution.
Their procedure for making these resins comprised reacting, in dioxane solution, an 8:1 mixture of an anhydrous polyethyleneimine-1800 molecular weight and a polymethylene polyphenylisocyanate to form a solid polyamine-polyurea crosslinked precursor. This, in turn, was reacted with a mixture of NH.sub.4 OH and CS.sub.2 in isopropyl alcohol over a period of about 4 weeks to form resins having a sulfur content of about 18 percent and an equivalent Cu.sup.+2 capacity (milliequivalents of Cu absorbed/gram of dry resin from an aqueous solution at a pH of about 4.8) of between about 0.8 and 1.35. If followed rigorously, the above procedure for making such resins possesses a number of serious disadvantages, in terms of producing the large quantities needed for use in industrial applications. For example, the 4-5 week total reaction time imposes significant costs on the production cycle.
However, more recent work by Miyazaki and Barnes of "Complexation of Some Transition Metals, Rare Earth Elements, and Thorium with a Poly(dithiocarbamate) Chelating Resin" in Analytical Chemistry, Vol. 53, No. 2, February 1981, pages 299-304, have shown that NH.sub.4 OH/CS.sub.2 reaction time can be reduced to as short a time as 8-16 hours with essentially equivalent results. Secondly, it was shown that both lower molecular weight polyimines and a variety of polyisocyanates can be used.
The common elements in these prior art studies are the use of dioxane as the solvent and an anhydrous polyamine used as a precursor reactant. Dioxane is a federally listed health hazard and it would be highly desirable if less hazardous solvents could be used. It is known that polyethyleneimines can be supplied as aqueous suspensions at attractive prices. However, they are, at best, only sparingly soluble in dioxane and attempts to form a satisfactory precursor resin from such a material almost invariably end in failure. The reason for this appears to be that the water in the suspension saturates the dioxane thus effectively inhibiting its ability to dissolve the polyimine so that very little, if any, is available to react with the polyisocyanate. Further, it is also known that the reaction forming the precursor need not be limited to isocyanate crosslinkers.